The present invention relates to a charge and discharge control circuit for controlling for charge and discharge of a secondary cell by controlling the ON/OFF state of a switch circuit and having a function of detecting charge and discharge current, and a chargeable power supply unit using the circuit.
The circuit block diagram of the prior chargeable power supply unit comprising a secondary cell is shown in FIG. 2. A secondary cell 101 is connected to an external terminal 105 or 104 through a switch circuit 103 and a sensing resistance 128. One terminal of the sensing resistance 128 and the switch circuit 103 is connected to a sensing terminal 129. A charge and discharge control circuit 102 is connected to the secondary cell 101 in parallel. The charge and discharge control circuit 102 has a function of detecting a voltage of the secondary cell 102. A signal is output from the charge and discharge control circuit 102 so that the switch circuit 103 turns OFF in any of the following states of the secondary cell 101: an over-charge state in which the voltage of the secondary cell is higher than a predetermined voltage, hereafter, this state is called an "over-charge protection state"; and an over-discharge state in which the voltage of the secondary cell 101 is lower than a predetermined voltage, hereafter, this state is called an "over-discharge protection state". By stopping discharge when the voltage of the sensing terminal 129 or the external terminal 104 reaches a certain voltage, it is possible to limit current flowing through the switch circuit 103. That is, it is possible to stop discharge or to control over-current when excessive current flows, hereafter, this state is called an over-current protection state.
The chargeable power supply unit has a function of detecting charge current and discharge current to the secondary cell. Because the sensing terminal 129 is connected to the secondary cell 101 in series, the charge and discharge control circuit 102 can detect current by monitoring the voltage at the sensing terminal 129. As sensing charge and discharge current results in monitoring cell capacity of the secondary cell, representation of remaining cell capacity is realized.
A circuit block diagram of an another example of the chargeable power supply unit comprising the prior secondary cell is shown in FIG. 3. In the circuit, the sensing resistance 128 shown in FIG. 2 is connected to a positive polarity 110 of the secondary cell in series. The remaining construction and parts in the circuit operate entirely similar to the above-mentioned circuit.
However, the prior charge and discharge control circuit constructed in the above-mentioned manner has the following defects in the case of detecting low charge and discharge current.
While charging to the secondary cell 101, current flows to the sensing resistor 128 from a charger 108 through the secondary cell 101. At this time, voltage of the sensing terminal 129 is lower than the negative polarity 111 of the secondary cell 101. Alternatively, while discharging, current flows to a load 109 from the secondary cell 101 through the sensing resistor 128. Voltage of the sensing resistor 129 is higher than the negative polarity 111 of the secondary cell. That is, the sensing resistor 128 (Rsens) multiplied by charge and discharge current value appears at the sensing terminal 129. By monitoring and calculating the value, the remaining capacity of the secondary cell can be monitored.
Being connected to the secondary cell 101 in series, the sensing resistor 128 should be small in resistance. This is because current is wasted during charge and discharge of the secondary cell when the sensing resistor has a large value. The sensing resistor therefore has value of 10 to 50 m.OMEGA..
However, when the sensing resistor has a small value, a problem occurs in the circuit. For example, assuming the following conditions: a discharge current of 100 mA; and a sensing resistor a 30 m.OMEGA.. At this time, the voltage at the sensing terminal 129 is only 3 mV. Although the voltage is generally amplified as voltage detected is low, it is difficult to measure correctly a voltage such as 3 mV because the offset voltage (Voffset) of the operational amplifier has a value of about 1 to 10 mV.
In order to calculate the remaining capacity of the secondary cell using a microcomputer, the voltage is applied to an A/D (Analog to Digital) converter. As a reference voltage of the A/D converter is generally about 3 V, the value of 3 mV must be amplified 400 times. Because of that, if the amplifier has Voffset, the output voltage includes an error. For example, the operational amplifier has an offset voltage of about 2 mV, the output includes an error of 0.8 V at 400 times multiple of input in the operational amplifier.
Although it is possible by using a technique such as trimming to reduce the offset voltage of an operational amplifier, the manufacturing process becomes extremely complex, and the product itself becomes expensive.
In order to solve the prior problem, an object of the present invention is to provide a configuration in which the resistance value of the sensing resistor is made large when charge and discharge current flowing through the secondary cell becomes small, and the resistance value of the sensing resistor is made small when the charge and discharge current flowing through the secondary cell becomes large. The terminal voltage of each sensing resistor at time is monitored by the operational amplifiers different from each other in amplification factor so as to receive no influence of offset voltage included in the operational amplifier. By this configuration, a charge and discharge control circuit is provided, which has a function of monitoring correctly charge and discharge current and which is high in performance, small in loss, and safe in use.